A few thoughts on the diamondiferous nature of lamprophyres from Wajrakarur Kimberlite Field, Southern India

The field photographs, salient features and data sources are shown in Figure 2 and Table 1. The Korrakodu lamprophyre (KKL) is emplaced into younger Venkatapuram granitoids within the Peninsular Gneissic Complex (PGC). The lamprophyre is melanocratic showing porphyritic texture along with granitic xenoliths (Fig. 2a). The KKL suggests asthenospheric overprint on lithospheric mantle source with prominent indications for carbonatite metasomatism (Raghuvanshi et al., 2019).

Figure 2.

Field photographs of lamprophyres of WKF. (a) Korrakodu (Raghuvanshi et al., 2019), (b) Kadiri (Pandey et al., 2018), (c) Sivarampet, (d) Kalagalla, (e) Udiripikonda (Pandey et al., 2017a, 2017b), (f) Khaderpet, (g) Mudigubba, (h) Pipe-8 (Lattavaram). See text for photo sources. WKF, Wajrakarur Kimberlite Field.

Three lamprophyre dikes occur at SSW of Kadiri (KDL), within the Kadiri Schist Belt. The rocks show porphyritic-panidiomorphic texture. Owing to their mineralogy, the rocks show calc-alkaline character (Fig. 2b). The dykes are free from xenoliths and exhibit mild deformation (Pandey et al., 2018).

Two lamprophyre dikes (MGL) occur at NNW side of Mudigubba (Kaul & Sisodia, 1976). The Sivarampet lamprophyre (SPL) occurs as a small intrusion emplaced within the PGC. The intrusion has no linear extensions confining to a small area and hence presumed to be a plug (Phani et al., 2018). The rock shows porphyritic-panidiomorphic texture (Fig. 2c). The lamprophyre exists in close proximity to Pipe-16 and Pipe-17 of Pennahobilam (Phani et al., 2023). Olivines are replaced by calcite with serpentinised rims. The rock shows glomeroporphyritic texture (Pankaj et al., 2020).

Thin dikes of lamprophyre occur at Kalagalla (KGL) emplaced in the greenschist rocks of Ramagiri-Penakacherla Schist Belt exposed in the walls of an old mine shaft excavated for gold mining. The boulders from excavation are piled upon surface (Fig. 2d) (Phani et al., 2020). The fine-grained rock shows pitted surface, the mineralogy and texture are obscured by metamorphism. The rock is classified as a calc-alkaline variety, spessartite (Phani et al., 2020).

A discontinuous lamprophyre dike is exposed towards west of Udiripikonda (UKL), emplaced within the granite-gneisses of the PGC (Ravi et al., 1998). The lamprophyre carries abundant clinopyroxene macrocrysts and is classified as calc-alkaline variety (Fig. 2e). Although this lamprophyre is co-spatial and coeval with the Wajrakarur kimberlites, the Udiripikonda lamprophyre shows a highly enriched mantle source that was modified by asthenospheric upwelling during the Mesoproterozoic (Pandey et al., 2017b).

The Khaderpet intrusion (KUL) is a unique association of ultramafic lamprophyre of aillikite affinity associated with carbonatite (sövite) (Chatterjee et al., 2008; Smith et al., 2013). The ultramafic rock crops out as a brecciated rock with angular fragments of crustal rocks resembling carapace of a tortoise (Fig. 2f). The cementing material of breccia is of ultramafic lamprophyric character on the surface; however, it grades into kimberlitic nature at depth which is confirmed by drilling (CRAE, 2004). The cementing material is dominated by olivine that is serpentinised and chloritized. The matrix to the breccia consists of calcite or saponite pseudomorphs after olivine and fine-grained titanite in a background of saponite-carbonate or serpentine. The rock contains pelletal lapilli made up of olivine pseudomorphs with rare magnesio-chromite at the core, and a matrix of saponite rimmed by fine titanite (Smith et al., 2013).

The Mudigubba lamprophyre (MGL) dikes show typical porphyritic-panidiomorphic texture with well-developed hornblende crystals (Fig. 2g). The MGLs are free from crustal xenoliths and proposed to be products of the post-collisional extension of the SCLM that was previously modified by subduction-related fluids (Pandey et al., 2017a).

The pipe-8 (P-8) of Lattavaram is of oval shape in surface morphology covering 0.5 ha area. The pipe is emplaced in the Archaean granitoids along EastNorthEast-WestSouthWest (ENE-WSW) fault that displaces Marutla Dome. The outcrop is expressed in scattered boulders of weathered serpentinised and carbonated melanocratic outcrops peeping through calcrete/soil cover for 1 m after which, hard and compact ultramafic lamprophyre is found. This pipe rock, classified as a Group-I kimberlite earlier, is now reclassified as an ultramafic lamprophyre owing to the presence of Ti-rich phlogopite, Ti-rich schorlomite garnet, high Na/K content in amphibole, Al- and Ti-rich diopside, a titanomagnetite trend in spinel and the presence of groundmass carbonates (Dongre et al., 2019). The rock contains ubiquitous olivine pseudomorphs with ferruginous stainings, crustal granite fragments giving rise to brecciated appearence (Fig. 2h).

Diamond indicator minerals or kimberlite indicator minerals (DIMs or KIMs) in non-kimberlitic volcanic rocks could be the same as those present in kimberlites i.e., pyrope garnet, magnesiochromite; more rarely picroilmenite and Cr-diopside. Archean volcanic rocks such as lamprophyres can also contain these minerals in very small proportions (Kaminsky, 2007). KIMs are indicator minerals collected from surface samples such as stream sediments or loam which guide on presence of kimberlite while DIMs are same mineral species extracted from a known or identified kimberlite or LCR. DIMs act as path finders and guide the presence of exposed or concealed diamond source rocks or kimberlites in an area (Benz, 2006; McClenaghan, 2005; McClenaghan & Kjarsgaard, 2007; Muggeridge, 1995). Lamprophyres of WKF also recorded DIMs such as olivine, Cr-diopside, ilmenite, spinel, garnets and diamonds. In addition, few lamprophyres reported to contain titanite with high concentrations of light rare earth elements (LREE). However, in India, attempts to study the full suite of DIMs in lamprophyres of WKF or elsewhere, focused to assess their diamond potentiality through heavy mineral assemblages has not been exercised. Thus, data specific to KIMs in WKF lamprophyres is meagre or absent. Therefore, some insights from published literature pertaining to mineral chemistry available in some of the WKF lamprophyres is summarised here. Presence of DIMs in comparison with lamprophyres and kimberlites of WKF is shown Table 3. The DIMs in kimberlites and lamproites of WKF are studied in detail by various workers in diamond exploration (CRAE, 2004; Ravi et al., 2016). The results indicate that they can be very well used in depicting the possible diamond incidence and is demonstrated physically through assessment of drill core. Interestingly, in KUL and P-8 lamprophyres, diamonds are also reported, offering a wide scope to investigate lamprophyres at other locations of the WKF.

The DIMs in lamprophyres warrant a thorough trained eye as they are sparse and difficult to identify than those in kimberlites and lamproites. The identification of the DIMs is on one hand crucial aspect while on the other, confirming whether the lamprophyre is diamondiferous which requires detailed mineral chemistry of these grains than a traditional visual identification is also vital. The composition of DIMs plays a critical role in their classification than a traditional visual identification. Hence, comprehensive studies to recover, visually classify, determining their chemical composition through electron probe micro-analyser (EPMA) and their appropriate mineral classification in terms of diamond potentiality are certainly inevitable.

Most natural diamonds are formed under high pressure within mantle rocks such as kimberlites and/or lamproites, where they interact with a deep-seated carbon-rich source (Shirey et al., 2013). Many factors influence the economic potential of LCRs including: (1) DG (i.e., the quantity of diamonds in a mass of the rock); (2) diamond quality, largely a combination of average diamond size recovered, clarity and colour; (3) size of the intrusive body; (4) local environment, including climate, local infrastructure and political stability (Kjarsgaard, 2007). With these complex factors, the estimation of the economic potential of a diamondiferous body is a very challenging task (Kjarsgaard, 2019).

The four Victory-Day dykes in Canada, pyropes of lherzolitic association (G9) and few picroilmenites and Cr-diopside were reported which led to the discovery of diamondiferous lamprophyres (Kaminsky et al., 2002). In the same area, the Thirsty Lake dyke samples revealed good results of diamond incidence- a 22 kg rock sample yielded 1509 micro- and 2 macrodiamonds; a 32.8 kg sample yielded 1157 micro- and 6 macrodiamonds and a 7.8 kg sample revealed 6677 micro- and 3 macrodiamonds with one stone of >1 mm in size. The average stone size varies from 0.8 mm to 0.5 mm with total estimated grade as 12 ct/ton. The Kyle Lake pipes, James Bay Lowlands, northern Ontario, suspected to be a type of lamprophyres, reported 0.6 ct/ton from 6.22 kg of rock sample. Aillikites in the Torngat Mountains, northern Quebec, recovered 2227 (>1 mm size) stones with 114 macrodiamonds (Kaminsky, 2007).

In India, comprehensive efforts to investigate the presence of DIMs and in turn diamonds in lamprophyres are yet to be exercised. The diamond potential of Gondwana lamprophyres and minnette of Jharia and Ranigunj was discussed (e.g., Basu et al., 1997; Rock et al., 1992). It should be admitted that no attempts for drilling the lamprophyres in particular, have been carried out for their detailed assessment. Therefore, it is very much necessary to carry out drilling followed by detailed assessment to test their diamond potentiality with the aid of latest diamond extraction methods such as caustic fusion. However, the KUL and the reclassified P-8, contain diamonds. The KUL yielded 58 stones from 485.8 kg of drill core material through caustic fusion process (CRAE, 2004). An assessment of a total of 1773 tonnes of rock material from 10 shallow pits over P-8 ultramafic lamprophyre has yielded 40 diamonds weighing 5.95 carats. This implies to ~0.5 cpht (Ravi et al., 2016; Subba Rao, 1992).

The WKF is a complex region where a variety of mantle derived diamondiferous alkaline, ultramafic, ultrapotassic rocks are emplaced. Extensive research and exploration have been carried out on diamondiferous kimberlites by various public and private organisations. However, no attempt has been made so far to assess the diamond potential of lamprophyres of the WKF or elsewhere in the country. The research carried out by many workers so far was focused on understanding the petrology, geodynamics and genetic significance to decipher the mantle systematics.

The whole rock geochemical data and cationic wt. % data are shown in Table 2 in Table 2a and 2b respectively. The WKF lamprophyres are mostly calc-alkaline in character. The WKF lamprophyres are abundantly enriched in biotite and thus show elevated K₂O content. The SiO₂ vs MgO and Al₂O₃ vs TiO₂ variations clearly show a mixed affinity to the global alkaline and calc-alkaline lamprophyres perhaps due to hybridisation. The Mg# ranges from 60 to 84 in WKF lamprophyres. The WKF lamprophyres host olivine and oscillatory zoned clinopyroxene phenocrysts that are silica-undersaturated. These clinopyroxenes show variable Ti content leading to their transitional or mixed orogenic - anorogenic geochemical behaviour, which is also reflected by their bulk-rock geochemistry. The U-Pb isotopic data of zircon using (Secondary Ion Mass Spectrometry (SIMS) provided an emplacement age 2511 ± 6.6 Ma for Udiripikonda lamprophyre, while the basement granites have an age of ~2.5 Ga (Khan et al., 2019). As the Udiripikonda dykes possess no cross-cutting relation, they are presumed to be coeval as they are emplaced in the vicinity of kimberlites, which is consistent with whole-rock Rb-Sr isochron age of 0.96 Ga. Trace element relations between Th/Yb, Nb/Yb, Mo/Pr, Ce/Pb and Nb/U along with abundant TiO₂ content in these calc-alkaline lamprophyres show their affinity to the ocean island basalts (OIBs) (Khan et al., 2019; Pandey et al., 2017b).

In due course of prospecting, if any LCR is discovered, its DIM populations are to be assessed for their chemistry which is used to envisage the diamond potential and in turn diamond incidence of each lithological facies is ascertained through analysis of samples collected from multidirectional drilling. The drill core or chip samples are subjected to latest methods like ‘caustic fusion’ to determine the diamond incidence in each facies (Rombouts, 2003). As it is a costly affair, many exploration geologists adopt geochemical approach for an initial diamond assessment. A number of geochemical components such as K₂O/Na₂O ratio (Krutoyarsky, 1959), the elevated concentrations of Cr and Mg and the decreased concentrations of Ti, Fe, and Al were considered as gauges of diamond potentiality of an LCRD KIMs such as magnesiochromite, Cr-diopside, pyrope, olivine, picroilmenite etc. in the LCRs (Muggeridge, 1995).

There are several geochemical indices which were used to predict diamond incidence. Exploration geologists adopt several techniques to envisage the diamond potentiality of positive catchment areas and interesting geological samples from the preliminary prospecting. Mantle xenocryst minerals which are called as KIMs are most common material to assess diamond potentiality of a catchment where stream or gully sediments are collected. The positive mineral chemistry results through EPMA represented by presence of KIMs plotting in ‘diamond inclusion’ field reflect initial diamond incidence (Oliver et al., 2011; Rombouts, 2003).

Certain elements such as Ni, Cr, Ba, Co, Sr, Rb, Nb, Mg, Ta, Ca, Fe, K, Ti and REE indicate the presence of kimberlite or LCR in a terrain (McClenaghan & Kjarsgaard, 2007). Birkett et al. (2004) had used trace elements to distinguish ‘prospective’ and ‘non-prospective’ kimberlites. A minimum of 45 ppm of Nb in soil indicates possible presence of a kimberlite in the near vicinity (Phani, 2019).

The cationic wt.% and the geochemical data were plotted in various geochemical diagrams to ascertain the diamondiferous nature of the lamprophyre rocks of the study area. The bivariant diagram between K and Al shows that KUL and P-8 samples are ‘diamondiferous’ while the rest of the lamprophyre samples plot in the ‘few + no diamonds’ field (Fig. 3a). In the Ti vs K diagram, in addition to KUL and P-8 samples, Mudigubba samples also plot in the ‘diamondiferous’ field while the rest of the samples plot in the ‘few + no diamonds’ field (Fig. 3b). In the Ti vs Al diagram the majority of the samples plot in the ‘few + no diamonds’ field except KUL and P-8. The MGL samples position in the border of ‘diamondiferous’ and ‘few + no diamonds’ fields reflecting possibility of diamond incidence. (Fig. 3c). In the K vs Fe diagram, majority of the WKF lamprophyres plot in the ‘few + no diamonds’ field. However, KUL and P-8 samples along with Mudigubba samples show ‘diamondiferous’ character (Fig. 3d). Even though the Jharia lamproite samples are reported to be diamondiferous, they plot in the ‘few + no diamonds’ or ‘no diamonds’ field in these diagrams.

Figure 3.

Geochemical diagrams plotted using cationic wt.% (Ilupin & Lutz, 1971; Ilupin et al., 1974). (a) K vs Al diagram, (b) Ti vs K variation diagram, (c) Ti vs Al diagram, (d) K vs Fe diagram. Data used for comparison: Wajrakarur diamondiferous kimberlites (Phani & Raju, 2017), Narayanpet barren kimberlites (Dongre and Chalapathi Rao, 2012), Wawa diamondiferous lamprophyre (Lefebvre et al., 2005).

It can be observed from the cationic wt.% diagrams that majority of kimberlites plot in the ‘diamonds’ field, along with diamondiferous lamprophyres of Wawa while two kimberlites namely P-5 and P-13. The barren kimberlites of Narayanpet clearly plot in the ‘no diamonds’ field. This reflects that these plots are quite useful in assessing the possible diamondiferous nature of WKF lamprophyres.

In the Ta vs Sc plot, the KUL and P-8 samples plot in the field of ‘diamondiferous Al-kimberlites while rest of the samples are outside the prospective field (Fig. 4a). This variation diagram differentiates all other lamprophyres from KUL and P-8 ultramafic lamprophyres. The Narayanpet kimberlites plot in the ‘Fe-Ti barren kimberlites’ field.

Figure 4.

Geochemical diagrams showing diamondiferous character of WKF lamprophyres. (a) Ta vs Sc variation diagram (Kaminsky et al., 2005) showing lamprophyres in comparison with other rock types. (b) Fe₂O₃ (wt.%) vs Y (ppm) diagram showing distinction between diamondiferous (prospective) and barren (non-prospective) kimberlites (Birkett et al., 2004). (c) Bivariant diagram between DG and TiO₂ (wt.%) for lamprophyre samples of WKF showing inverse relation. Data for WKF kimberlites from Phani (2019). DG, diamond grade; WKF, Wajrakarur Kimberlite Field.

The Y vs Fe₂O₃ diagram shows that lamprophyre samples of KUL and P-8 plotting in the ‘prospecting’ zone (Fig. 4b). The diamondiferous kimberlites of WKF also plot in the ‘prospective’ zone. The Narayanpet kimberlites plot in the ‘non-prospective’ zone.

The ratio of cationic weight percentages of iron and titanium (Fe:Ti) plays a major role in diamond content of a kimberlite (Milashev, 1965). The DG values calculated for the WKF lamprophyres are shown Table 3. The petrochemical criterion for diamond is mainly governed by the inverse correlation of TiO₂ and FeO_T. Higher the TiO₂, lower is the diamond content (Krivonos, 1999; Milashev, 1965). This is also evident in the lamprophyres of the present study. The TiO₂ vs DG variation diagram clearly reflects that in KUL, MGL and KDL samples, the DG values are high (>5) with relatively less TiO₂ content (Table 4). Thus, the bivariant plot between DG and TiO₂ shows a strong inverse relation (r = −0.84). As TiO₂ is increased, the DG values decrease (Fig. 4c). This aspect is consistent with the results of whole rock geochemistry of WKF kimberlites (Chalapathi Rao & Srivastava, 2009).

Chalapathi Rao (2008) opined that lamprophyre of PAP are non-diamondiferous due to their shallow derivation, primitive and evolved nature. In case of lamprophyres and olivine lamproites of Bokaro Coalfields also, rarity of harzburgite nodules and absence of garnets reflect shallower source (Basu et al., 1997) while Jharia minnette have reported diamonds (Rock et al., 1992. However, the WKF lamprophyres possess a different geochemical signature and have cogenetic origin with kimberlites of the region. The SPL is derived from a shallow (~100 km) depth which is supported by absence of garnet. Yet, the presence of extractable diamonds in KUL and P-8 lamprophyres infers that lamprophyre magmatism in this region has deeper source regions similar to diamondiferous kimberlites of this region. The presence of diamonds in KUL and P-8 lamprophyres implies their petrological significance representing closure of a magmatic cycle in this igneous domain (Rock, 1977). The lamprophyres of EDC share similar petrogenetic processes to those of kimberlites and lamproites (Chalapathi Rao, 2008). The observations of this study using geochemical plots, DG values and field exploration results, it can be stated that the WKF lamprophyres deserve a comprehensive study in terms of their diamond potentiality. The presence of diamonds in KUL and P-8 also gives a clue on their deeper derivation and portrays an evolutionary history of underlying mantle that requires a revisit.